1. Field of the Invention
This invention relates to a photoelectric encoder including a scale having an incremental track formed with reflection gratings at equal intervals along a length measurement direction and a detector which displaces relative to the scale and in particular to a photoelectric encoder which circumvents erroneous detection of an origin because of diffraction of light in an origin mark and can accurately detect an origin.
2. Description of the Related Art
For example, a configuration as shown in International Patent Publication No. 2008-503745 is proposed to provide a small detector configuration and an origin detection function in a photoelectric encoder including a scale having an incremental track formed with reflection gratings at equal intervals along a length measurement direction and a detector which displaces relative to the scale. In the configuration, an origin mark is put on the incremental track of the scale. Thus, in International Patent Publication No. 2008-503745, an image of light applied to the origin mark is formed on a sensor divided into two by image forming means of a lens, etc., and one output of the sensor is subtracted from the other output of the sensor. A zero cross position is found in the subtraction signal (differential signal), whereby an origin is detected in International Patent Publication No. 2008-503745.
However, in International Patent Publication No. 2008-503745, the detector is provided with at least the image forming means and thus it is feared that the detection may be complicated.
In the placement concerning origin detection in International Patent Publication No. 2008-503745, for example, it is also considered that light is reflected on the origin mark and the reflected light is made incident directly on the sensor without using the image forming means. However, the origin mark is considerably small and thus when light is applied to the origin mark, light is diffracted. Thus, the diffracted light caused by the origin mark is incident on the sensor and when the differential signal is obtained, more than one zero cross position occurs and it is feared that the origin may be erroneously detected.